System and method for individual particle sizing using light scattering techniques

ABSTRACT

A particle sizing system is provided that includes an optical source generating a light beam for illuminating particles in a monitored volume, a plurality of light deflectors, each positioned to receive and deflect light scattered by the particles, and an image capture device collecting scattered light deflected by each light deflector. The image capture device outputs an image including a plurality of sub-images, each generated from the collected light deflected from a respective one of the light deflectors. Each particle is imaged as a spot in each sub-image, the plurality of spots associated with each particle corresponding to a plurality of scattering angles. The system also includes a processing unit configured to identify the spots associated with each particle in the sub-images, compute a spot parameter associated with each spot, and determine the size of each particle from its related spot parameters. A particle sizing method is also provided.

TECHNICAL FIELD

The general technical field relates to particle sizing techniques and,in particular, to a system and method for individual particle sizingusing light scattering.

BACKGROUND

Airborne particulate matter (PM) is a growing concern worldwide and isknown to have an adverse impact notably on human health, on theenvironment and on climate change. Many large cities around the world gothrough frequent and/or extended periods of time during which PMconcentration levels exceed accepted thresholds. In 2014, the WorldHealth Organization estimated that ambient air pollution contributes to6.7% of all deaths worldwide. Studies have also established a linkbetween air pollution and strokes from correlations with PM measurementsperformed in large cities, while other studies have associated airpollution with autism and learning disabilities in young children.Airborne PM can have different origins and chemical compositions, andcan travel over long distances, so that regulations aimed at managingthem are getting more stringent and complex. Recent trends and advancesin environmental monitoring have also lead to new demands in terms of PMcontrol and management.

Particle sizing techniques based on light scattering are known and havebeen used in different fields and with different types of materials.Such techniques generally involve providing a sample of particles,illuminating the sample, measuring light scattered by the particles, andanalyzing the scattering measurements to obtain particle sizeinformation. Particle size distributions can also be obtained throughstatistical data accumulated over time on individual particles or byusing inversion methods applied to a representative population of thesample.

Several commercially available systems use light scattering fordetermining the size of particles, typically using a laser diode forproducing the light beam for particle illumination. The particles areusually supplied to a chamber by a vacuum-based pumping system thatsamples part of the ambient medium. The size of the chamber is typicallysmall, with sidewalls of the order of a few millimeters (mm) long. Thelight beam usually provides a uniform illumination of the particles toreduce measurement errors arising from the fact that different scatteredsignals may originate from particles illuminated by different portionsof the beam. An optical detector measures the amount of light scatteredfrom the particles, usually at a scattering angle of about 90° relativeto the propagation direction of the illumination light beam. Such a“sideway” scattering detection scheme may allow the particle sizingsystem to be made more compact and the amount of stray light reachingthe detector to be reduced.

It is known that the particle sizing systems discussed above have somedrawbacks and limitations. First, the intensity of light scattered at90° is generally quite sensitive to the composition of the particles. Asa result, proper calibration of the particle sizing system as a functionof particle composition is generally unavoidable to ensure meaningfulparticle size measurements. In addition, the vacuum-based pumpingsystems typically used with conventional particle sizing systems aresusceptible to mechanical wear and damage. As a result, these pumpingsystems generally require careful inspection and maintenance which, inturn, can substantially increase the operating costs of the particlesizing systems. Pumping systems also typically have to be calibrated toensure that the supply rate of particles to the chamber is known, sinceits value will affect the number of particles analyzed per unit of time.Yet another limitation of conventional systems comes from the fact thatparticles are sampled from the ambient medium and supplied to thechamber by the pumping system. The sampling process can cause differentmeasurement errors and biases due, for example, to inlets being biasedto a certain particle size, to particles breaking up as a result ofhitting system components, to particle deposition on wall surfaces, andthe like.

Other types of particle sizing systems have been developed where forwardrather than sideway scattered light is detected. In such systems, theintensity of the detected scattered signals may, in principle, be madeless sensitive to particle composition. However, these systems generallyrely on inversion methods such as mentioned above, which yield particlesize distributions rather than individual particle sizes and oftennecessitate a model or a priori knowledge of the particle composition tobe applied.

Accordingly, many challenges remain in the development of particlesizing systems and methods that use light scattering for determining thesize of individual particles in a sample, while also being lesssensitive to particle composition and involving less mechanicalmaintenance.

SUMMARY

In accordance with an aspect, there is provided a particle sizingsystem. The particle sizing system includes:

-   -   an optical source generating a light beam, the light beam        illuminating particles contained in a monitored volume;    -   a plurality of light deflectors, each light deflector being        positioned to receive and deflect light scattered by the        illuminated particles;    -   an image capture device collecting deflected scattered light        from each light deflector, the image capture device outputting        an image including a plurality of sub-images, each sub-image        being generated from the collected light deflected from a        respective one of the plurality of light deflectors, each        illuminated particle being imaged as a spot in each of the        plurality of sub-images, the plurality of spots associated with        each illuminated particle corresponding to light scattered at a        plurality of scattering angles; and    -   a processing unit receiving the image from the image capture        device, the processing unit being configured to, for each        illuminated particle, identify the plurality of spots associated        with the illuminated particle in the plurality of sub-images,        determine a spot parameter associated with each of the plurality        of spots, and determine a size of the illuminated particle from        the plurality of spot parameters.

In accordance with another aspect, there is provided an imaging modulefor use in a particle sizing system. The imaging module includes:

-   -   a plurality of light deflectors, each light deflector being        positioned to receive and deflect light scattered by particles        contained in a monitored volume and illuminated by a light beam;        and    -   an image capture device collecting deflected scattered light        from each light deflector, the image capture device outputting        an image including a plurality of sub-images, each sub-image        being generated from the collected light deflected from a        respective one of the plurality of light deflectors, each        illuminated particle being imaged as a spot in each of the        plurality of sub-images, the plurality of spots associated with        each illuminated particle corresponding to light scattered at a        plurality of scattering angles and being characterized by        respective spot parameters, a combination of the plurality of        spot parameters being indicative of a size of the illuminated        particle associated therewith.

In accordance with another aspect, there is provided an imaging moduleas described herein, in combination with a computer readable memorystoring computer executable instructions thereon that when executed by acomputer perform the steps of:

-   -   receiving the image acquired by the image capture device; and,        for each illuminated particle,    -   identify the plurality of spots associated with the illuminated        particle in the plurality of sub-images, determine the spot        parameter associated with each of the plurality of spots, and        determine the size of the illuminated particle from the        plurality of spot parameters.

In accordance with another aspect, there is provided a computer readablememory storing computer executable instructions thereon that whenexecuted by a computer perform the steps of:

-   -   receiving an image from an imaging module for use in a particle        sizing system, the imaging module including:        -   a plurality of light deflectors, each light deflector being            positioned to receive and deflect light scattered by            particles contained in a monitored volume and illuminated by            a light beam; and        -   an image capture device collecting deflected scattered light            from each light deflector, the image capture device            outputting an image including a plurality of sub-images,            each sub-image being generated from the collected light            deflected from a respective one of the plurality of light            deflectors, each illuminated particle being imaged as a spot            in each of the plurality of sub-images, the plurality of            spots associated with each illuminated particle            corresponding to light scattered at a plurality of            scattering angles and being characterized by respective spot            parameters, a combination of the plurality of spot            parameters being indicative of a size of the illuminated            particle associated therewith; and, for each illuminated            particle,    -   identify the plurality of spots associated with the illuminated        particle in the plurality of sub-images, determine the spot        parameter associated with each of the plurality of spots, and        determine the size of the illuminated particle from the        plurality of spot parameters.

In accordance with another aspect, there is provided a particle sizingmethod. The method includes the steps of:

-   -   illuminating particles contained in a monitored volume;    -   receiving and deflecting light scattered by the illuminated        particles with a plurality of light deflectors;    -   collecting and imaging deflected scattered light from each light        deflector with an image capture device;    -   outputting an image generated by the image capture device, the        image including a plurality of sub-images, each sub-image being        generated from the collected light deflected from a respective        one of the plurality of light deflectors, each illuminated        particle being imaged as a spot in each of the plurality of        sub-images, the plurality of spots associated with each        illuminated particle corresponding to light scattered at a        plurality of scattering angles; and, for each illuminated        particle,    -   identifying the plurality of spots associated with the        illuminated particle in the plurality of sub-images, determining        a spot parameter associated with each of the plurality of spots,        and determining a size of the illuminated particle from the        plurality of spot parameters.

Other features and advantages of the embodiments of the presentinvention will be better understood upon reading of preferredembodiments thereof with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are numerically calculated scattering cross sectioncurves plotted as a function of scattering angle for dust-like particlesof various diameters when illuminated with light at a wavelength of 532nanometers (nm). In FIG. 1C, each point represents an average over ascattering angle range of 1°.

FIGS. 2A and 2B are ratios of scattering cross sections calculated forvarious pairs of scattering angles (1.5° and 5°: solid line; 1.5° and25°: short-dashed line; and 5° and 25°: long-dashed line) plotted as afunction of the diameter of dust-like particles at a wavelength of 532nm. The angular coverage is 0.1° in FIG. 2A and 0.3° in FIG. 2B.

FIG. 3 shows a numerically calculated scattering cross section curvesplotted as a function of particle diameter for spherical quartzparticles at a wavelength of 532 nm. Each curve corresponds to adifferent scattering angle, ranging from 1° to 180°.

FIGS. 4A to 4D show numerically calculated scattering cross sectioncurves plotted as a function of scattering angle for three differentparticle compositions (carbonaceous, quartz and dust-like aerosolparticles) at a wavelength of 905 nm. Each of FIGS. 4A to 4D correspondsto a different particle diameter, namely FIG. 4A: 0.7 micrometers (μm);FIG. 4B: 1.5 μm; FIG. 4C: 5 μm; and FIG. 4D: 10 μm.

FIG. 5 is a schematic top view of a particle sizing system, inaccordance with an embodiment.

FIGS. 6A to 6C are schematic top views of a same particle sizing system,in accordance with another embodiment. Each of FIGS. 6A to 6Cillustrates the angular coverage of one of the plurality of lightdeflectors of the particle sizing system.

FIG. 7 is a schematic top view of a particle sizing system, inaccordance with another embodiment.

FIG. 8 is a schematic top view of two embodiments of a particle sizingsystem, the two embodiments sharing the same optical source and the sameprocessing unit.

FIG. 9 is a schematic top view of a particle sizing system, inaccordance with another embodiment, where the particle sizing system isadapted for use with personal protective equipment.

FIG. 10 is a schematic top view of a particle sizing system, inaccordance with another embodiment.

FIG. 11 is a schematic top view of a particle sizing system, inaccordance with another embodiment.

FIG. 12 is a schematic top view of a particle sizing system, inaccordance with another embodiment.

FIG. 13 is a schematic top view of a particle sizing system, inaccordance with another embodiment.

FIGS. 14A and 14B are respectively schematic top and side views of aparticle sizing system, in accordance with another embodiment.

FIGS. 15A and 15B are respectively schematic top and side views of aparticle sizing system, in accordance with another embodiment, whereinthe light beam illuminating the particles in the monitored volume is afan-shaped beam.

FIG. 16 is a schematic representation of an image acquired by theparticle sizing system of FIG. 5.

FIG. 17 is a flow chart of a particle sizing method, in accordance withan embodiment.

DETAILED DESCRIPTION

In the following description, similar features in the drawings have beengiven similar reference numerals, and, in order to not unduly encumberthe figures, some elements may not be indicated on some figures if theywere already identified in preceding figures. It should also beunderstood herein that the elements of the drawings are not necessarilydepicted to scale, since emphasis is placed upon clearly illustratingthe elements and structures of the present embodiments.

General Overview—Particle Sizing Using Light Scattering Techniques

The present description generally relates to techniques for determiningthe size of individual particles contained in a monitored volume. Inaccordance with different aspects, there are provided a particle sizingsystem, an imaging module for use in a particle sizing system, and aparticle sizing method. The present techniques generally use lightscattering to determine, in situ, information about the size ofindividual particles, but also, in some embodiments, about one or moreother particle characteristics including, without limitation, positionwithin the volume, shape, composition, phase and optical absorption. Insome implementations, the present techniques can also be used todetermine the particle size distribution of a collection of particles ina fluid medium from individual particle sizes measured over time oracross a large monitored volume.

The techniques described herein may be particularly useful in anyapplication where it is desired to determine the size of individualparticles contained in a host medium. By way of example, the presenttechniques may be used in air quality monitoring to determine theparticle size distributions and, in particular, the PM2.5 (PM up to 2.5μm size) and PM10 (PM up to 10 μm size) contents of airborne PM. Thepresent techniques may also be used for emission monitoring of processesgenerating various types of aerosols such as, for example, power plants.Other applications may include tailings pond monitoring and real-timecontrol of the size distribution of water droplets from water sprayersused to reduce dust released from mining operations (e.g., coal mining).The present techniques may further be used in process monitoring andcontrol of pharmaceutical drug production where the size of theparticulate drugs can affect the efficiency of the drug deliveryprocess. Yet another possible application includes quality assessment oftransparent solid materials (e.g., glass) to detect and determine thesize of bulk and/or surface defects that can scatter light.

As used herein, the term “particle” and any variant thereof referbroadly to any individual mass, structure or refractive indexnon-uniformity, or any collection thereof, that is capable of scatteringlight incident thereonto. It will be understood that, in principle, theterm “particle” is not meant to be restricted with respect to size,shape or composition. For example, some embodiments may be suited forsizing substantially spherical particles having a diameter ranging fromabout 0.1 μm to 100 μm and, in particular, from about 0.2 μm to 40 μm,although other shapes and sizes may be contemplated in otherembodiments. Both the particles and the host medium in which theparticles are suspended, dispersed, contained or otherwise located maybe gaseous, liquid or solid, as long as the particles and the hostmedium have different refractive indices. The host medium may betransparent or semi-transparent. By way of example, in some embodiments,the particle may be solid particles suspended in a gas or a liquidmedium. Unless otherwise specified, the term “particle size” and anyvariant thereof refer herein to the diameter of a particle.

As used herein, the term “scattering” and any variant thereof refer in abroad sense to the dispersal of light by one or more particles as aresult of physical interactions therewith. The mechanisms involved mayinclude, but are not limited to, reflection, refraction, absorption anddiffraction, as well as fluorescent, phosphorescent or luminescentphenomena. Depending on the particular physical process at play, thescattering may be accompanied or not by a wavelength shift of thescattered light with respect to the light illuminating the particles.

As used herein, the terms “light”, “optical” and any variant thereof areintended to refer to electromagnetic radiation in any appropriate regionof the electromagnetic spectrum and, in particular, are not limited tovisible light but may also include the terahertz, infrared andultraviolet ranges. By way of example, in some embodiments, the terms“light” and “optical” may encompass electromagnetic radiation with awavelength ranging from about 350 nm to 1000 nm.

It is known in the art that light scattering may be classified as“forward scattering”, “backward scattering”, and “sideway scattering”.The terms “forward scattering”, “backward scattering” and “sidewayscattering” refer to light scattered in a direction making an anglerespectively smaller than, greater than and close to 90° with thepropagation direction of the incident beam. As will be discussed furtherbelow, in some implementations, the particle sizing techniques describedherein may use forward scattering at small angles in order to eliminateor at least reduce the dependence of the particle size determination onthe refractive index (and thus on the composition) of the particle.

Referring to FIGS. 1A and 1B, it is known that for small scatteringangles, the forward scattering cross section at a given wavelengthgenerally decreases monotonically with the scattering angle. As usedherein, the term “small scattering angle” refers generally to scatteringangles which, when expressed in radians, are smaller than the ratio ofthe light wavelength to the particle diameter. It is also known that thelowest scattering angle at which the value of the angular scatteringcross section gets equal to half its maximum value at zero degree variesinversely with the particle diameter. These characteristics can make itpossible to achieve, in principle, particle sizing from the main forwardscattering lobe of a single particle, as described for example in J.Raymond Hodkinson, “Particle Sizing by Means of the Forward ScatteringLobe”, Applied Optics vol. 5, issue 5, pp. 839-844 (1966).

By way of example, in some embodiments of the present techniques, thesize of a particle may be determined by collecting forward scatteredlight at different scattering angles, and then using reference dataobtained from the Mie theory or from another appropriate lightscattering theory to determine the particle size that better matchesmeasurement data at the chosen light wavelength. As known in the art,the Mie scattering theory provides a complete angular distribution ofscattered light intensity from an isotropic and homogenous sphericalparticle as a function of its size, refractive index and wavelength ofthe incident light.

According to the techniques described herein, the amount of lightscattered by a particle is measured at two or more scattering angles. Insome implementations, one or more ratios may be obtained between the twoor more measured scattered signals. The one or more ratios may then becompared with reference data, for example theoretical ratios calculatedfrom the Mie theory, to determine the size of the particle.

It will be understood that implementing such an approach may involve acareful selection of the different scattering angles at which thescattered signals are to be measured at a given wavelength. By way ofexample, as seen in FIGS. 1A and 1B, for particles having a diameterranging from about 0.2 μm to 8 μm, the angular scattering cross sectionhas a monotonic behavior for scattering angles lower than about 5°.However, for particles larger than 8 μm, it is seen that the angularscattering cross section oscillates as a function of scattering angleeven at small scattering angles. These oscillations are caused byangle-dependent interference between waves scattered by differentportions of the particle. As seen in FIG. 1C, the amplitude of theseoscillations tends to be reduced by averaging each point of the curvesover a scattering angle range (or angular coverage) of 1°. It is to benoted that most particles released from industrial plants are generallynot perfectly spherical. In some implementations, the non-sphericalcharacter of the particles may have an averaging effect that canattenuate the oscillations of the angular scattering cross section.

Turning to FIGS. 2A and 2B, there are illustrated curves of the ratio ofscattering cross sections calculated for various pairs of scatteringangles, namely 1.5° and 5° (solid line), 1.5° and 25° (short-dashedline) and 5° and 25° (long-dashed line). Each ratio is plotted as afunction of the diameter of dust-like particles illuminated with lightat a wavelength of 532 nm. The angular coverage is 0.1° in FIG. 2A and0.3° in FIG. 2B. The larger angular coverage in FIG. 2B aims to simulatethe presence of an aperture in the optics used to collect the scatteredlight.

As seen in FIGS. 2A and 2B, the ratio between the scattered intensitiesat 1.5° and 5° (solid line) increases monotonically for diameters up toabout 7 μm, which makes it possible, in principle, to determine the sizeof particles smaller than 7 μm solely from this ratio. However, thepresence of oscillations for diameters larger than 7 μm may prevent orat least make it more difficult to properly determine particle size inthis range. Furthermore, since the ratio is nearly constant fordiameters between 0.1 and 1 μm, particles in this range may not bereadily discriminated from this ratio alone, but could be if one or bothof the ratios between the scattered intensities at 1.5° and 25°(short-dashed line) and the scattered intensities at 5° and 25°(long-dashed line) are considered in the analysis.

FIG. 2A also shows that for particles larger than 7 μm, some diameterranges would exhibit similar combinations of values for the threeratios, thus preventing or at least making it difficult for particlesizes to be determined with sufficient accuracy. In such a case, it isseen from FIG. 2B that the aperture of the collecting optics can providean averaging effect that tends to attenuate the oscillations in theratios as a function of diameter and that may alleviate at least some ofthe ambiguities observed in FIG. 2A. In particular, it is seen that theoscillations in the ratios are more strongly attenuated for largerparticles (i.e., over about 20 μm), thus reducing the range of particlediameters for which similar ratio values are observed and, in turn,improving the size discrimination capabilities.

Referring now to FIG. 3, there are illustrated calculated curves of theangular scattering cross section of spherical quartz particles plottedas a function of particle diameter at a wavelength of 532 nm, each curvecorresponding to a different scattering angle between 1° and 180°. It isseen that for small scattering angles (i.e., below 5°), the angularscattering cross section varies by up to three orders of magnitude whenthe particle diameter increases from 0.2 to 1 μm, by two to three ordersof magnitude between 1 et 10 μm, and by one order of magnitude or lessbetween 10 and 40 μm, for a total of six to seven orders of magnitudebetween 0.2 and 40 μm. Therefore, in some implementations, the strengthof the measured scattered signals at small forward scattering angles mayprovide an estimate of the order of magnitude of the particle size. Inother words, absolute values of the measured scattered intensities maybe used to get a coarse estimate of the particle size. Combining theresults obtained from this pre-classification with information gatheredfrom ratios of scattered intensities at one or more pairs of scatteringangles may allow particle size to be determined with enhanced accuracywhen measurement data is compared with reference data.

FIG. 3 also illustrates the benefit, in some implementations, ofcarefully selecting the different observation angles to allow the sizeof small particles to be determined with sufficient accuracy. By way ofexample, almost no signal differences would be perceived betweendifferent observation angles in the range from 1° to 10° in the case ofparticles having diameters from about 0.2 to 0.6 μm.

A further aspect that may affect the validity of particle sizing is thedependence on particle composition or, equivalently, the particlerefractive index. Referring to FIGS. 4A to 4D, there are illustratedcalculated curves of the angular scattering cross section plotted as afunction of scattering angle for three different particle compositions(carbonaceous, quartz and dust-like aerosol particles) at a wavelengthof 905 nm, each of FIGS. 4A to 4D corresponding to a different particlesize (diameter of 0.7 μm, 1.5 μm, 5 μm and 10 μm, respectively). Thecurves depicted in FIGS. 4A to 4D have been calculated assuming asingle-mode log-normal distribution of particles to avoid or at leastattenuate the effect of oscillations at large scattering angles.

As seen in FIG. 4A, the angular scattering cross section of smallerparticles (i.e., smaller than the light wavelength) can dependnon-negligibly on particle composition in the whole range of scatteringangles between 0° and 180°. More specifically, it is seen that forsmaller particles, strongly absorbing particles scatter light lessstrongly than weakly absorbing ones. However, the relative angularbehavior generally does not depend significantly on particle compositionfor the main diffraction lobe (i.e. below 90°). Therefore, using bothabsolute and relative values of measured scattered signals to determineparticle size may be useful for smaller particles whose composition isnot known a priori.

Meanwhile, FIGS. 4B to 4D indicate that the angular scattering crosssection of larger particles (i.e., those larger than the lightwavelength) remains largely independent of particle composition at smallforward scattering angles (i.e., between about 1° and 5°). Accordingly,in some implementations, relations (e.g., ratios) between forwardscattered signals measured at small scattering angles may be used todetermine or at least estimate the size of individual particles withoutrequiring prior knowledge of their composition. By contrast, FIGS. 4A to4D also illustrate that particle composition may have a significanteffect on the capability of determining particle size as the scatteringangle increases. This can explain, at least partly, why existingparticle sizing systems based on sideway scattering measurementsgenerally need to be calibrated as a function of particle composition.

Particle Sizing System

Referring generally to FIGS. 5 to 15B, various exemplary embodiments ofa particle sizing system 20 for determining the size of particles 22present in a monitored volume 24 are illustrated. The particles arehereinafter referred to generally and collectively as 22, butindividually as 22 ₁ and 22 ₂. Broadly described, and as discussed ingreater detail below, the particle sizing system 20 generally includesfour main components, namely an optical source 26, a plurality of lightdeflectors 28 a to 28 c, an image capture device 30, and a processingunit 32.

The optical source 26 generates a light beam 34 that then propagatesalong an optical axis 36, the light beam 34 illuminating particles 22contained in the monitored volume 24. As used herein, the term“monitored volume” refers to a region of space containing particlesilluminated by the light beam and defined such that light scattered fromeach illuminated particle in the region of space is collected at aplurality of distinct scattering angles after deflection from theplurality of light deflectors. In other words, the monitored volumerepresents the portion of the light beam that can be imaged by the imagecapture device via each one of the plurality of light deflectors.Accordingly, it will be understood that the monitored volume is definednot only by the cross-sectional area of the light beam, but also by theposition of each light deflector in the field of view of the imagecapture device. In this regard, it is noted that the term “position” andany derivative thereof refer herein to the full positional informationof an object in space, including both location and orientationcoordinates.

It is noted that, in some cases, particle size information may beobtained for illuminated particles that are located outside of the“monitored volume” as defined herein, but whose part of scattered lightis collected by the image capture device via at least one of theplurality of light deflectors. By way of example, in an implementationwhere particle size can be determined from ratios of scatteredintensities at pairs of scattering angles, the size of a given particlemay be determined solely from scattered light collected by the imagecapture device via two light deflectors, even if the particle sizingsystem includes more than two such light deflectors.

Generally, the illuminated particles 22 in the monitored volume 24 causethe light beam 34 to be scattered, in all directions. Each lightdeflector 28 a to 28 c is positioned to receive and deflect scatteredlight 38 in a certain scattering angle range from each illuminatedparticle 22 in the monitored volume 24. As mentioned above, in someembodiments, the light deflectors 28 a to 28 c may be configured todeflect forward scattered light onto the image capture device 30. Morespecifically, in such embodiments, each light deflector 28 a to 28 c maybe positioned such as to deflect a respective forward scatteredcomponent of the light 38 scattered from each particle 22 present in themonitored volume 24. In other words, for each particle 22, thescattering angle range covered by each light deflector 28 a to 28 c islimited to scattering angles that are smaller than 90° relative to thepropagation direction of the light beam 34 along the optical axis 36. Byway of example, in some embodiments, the forward scattered componentdeflected by each light deflector 28 a to 28 c represents light 38scattered at an angle smaller than 35° with respect to the propagationdirection of the light beam 34.

The image capture device 30 collects deflected scattered light 40 withinits field of view from each one of the plurality of light deflectors 28a to 28 c. For this purpose, the image capture device 30 can includecollecting optics 46 adapted to collect deflected scattered light 40from the plurality of light deflectors 28 a to 28 c. The image capturedevice 30 also outputs an image including a plurality of sub-images,where each sub-image is generated from the collected light deflectedfrom a respective one of the plurality of light deflectors 28 a to 28 c.Each illuminated particle 22 is imaged as a spot in each sub-image, suchthat the plurality of spots associated with a given particle 22corresponds to light scattered by this particle 22 at a plurality ofdifferent scattering angles.

The processing unit 32 then receives the image from the image capturedevice 30. As used herein, the term “processing unit” denotes an entityof the particle sizing system that controls and executes, at leastpartially, the functions required to operate the particle sizing systemincluding, without being limited to, determining particle size from theimage acquired by the image capture device. The processing unit 32 maybe implemented as a single unit or as a plurality of interconnectedprocessing sub-units. Also, the processing unit 32 may be embodied by acomputer, a microprocessor, a microcontroller, a central processing unit(CPU), or by any other type of processing resource or any combination ofsuch processing resources configured to operate collectively as aprocessing unit. The processing unit 32 may be provided within one ormore general purpose computers and/or within any other suitablecomputing devices. Also, the processing unit 32 can be implemented inhardware, software, firmware, or any combination thereof, and beconnected to the various components of the particle sizing system 20 viaappropriate communication ports.

For each illuminated particle 22, the processing unit 32 is firstconfigured to identify the plurality of spots associated with theparticle 22 in the plurality of sub-images. It will be appreciated thatwith proper knowledge or calibration of the relative positions of thelight beam 34, the light deflectors 28 a to 28 c and the image capturedevice 30, it may be possible to associate a location in the monitoredvolume 24 with each spot visible in each sub-image. In other words, itis possible to determine where the light scattered by each illuminatedparticle 22 at a particular location in the monitored volume 24 willform a spot in each of the sub-images after deflection from thecorresponding one of the light deflectors 28 a to 28 c.

The processing unit 32 is also configured to determine a spot parameterassociated with each of the plurality of spots, and to determine a sizeof the illuminated particle 22 from the plurality of spot parameters. Insome implementations, each spot parameter may be an energy parameterindicative of an amount of optical energy contained in the spotassociated therewith. In other implementations, the spot parameters maycorrespond, without limitation, to a size, a shape, a polarization or aspectral content of the spots, or any combination thereof. When the spotparameters are energy parameters, the processing unit 32 may beconfigured to calculate or otherwise obtain one or more ratios betweenthe energy parameters associated with each illuminated particle 22. Insuch a case, the size of each illuminated particle 22 may be determinedfrom a comparison of the one or more ratios with reference data. By wayof example, the reference data can be obtained from a numerical oranalytical model based on the Mie scattering theory or on anothersuitable theoretical framework (e.g., the Rayleigh or Fraunhoferscattering theories) allowing a reference scattering response of theparticles to be obtained. In particular, the manner of obtaining thereference data is not meant to limit the scope of application of thetechniques described herein.

It is worth mentioning that the techniques described herein may employrelative signals (e.g., signal ratios) rather than absolute signals. Insome embodiments, the use of relative signals can make the particlesizing system 20 less sensitive to uniformity fluctuations in thetransverse irradiance profile of the light beam illuminating theparticles. Therefore, in such a case, because ratios of scatteredsignals are used for particle sizing, the beam of light may not need tofulfill specific requirements in terms of uniformity and/or powerstability. Also, an illuminated particle 22 may be located anywhere inthe monitored volume 24 without significantly degrading the validity ofthe size determination.

It is also to be noted that, in some implementations, the lightdeflectors 28 a to 28 c and the image capture device 30 can define animaging module 42 for use in the particle sizing system 20. In theexemplary embodiments presented below, the imaging module 42 is depictedand described as forming part of the particle sizing system 20. However,it will be understood that, in other embodiments, the imaging module 42may be manufactured and sold as a separate integrated unit for use in aparticle sizing system with other components, for example, but notlimited to, an optical source and a processing unit. However, in eithercase the imaging module 42 includes the plurality of light deflectors 28a to 28 c positioned to receive and deflect light scattered 38 byilluminated particles 22 contained in the monitored volume 24 andilluminated by the light beam 34, and the image capture device 30 forcollecting deflected scattered light 40 from each of the lightdeflectors 28 a to 28 c.

In some implementations, the imaging module 42 may be used incombination with a computer readable memory 43 storing computerexecutable instructions thereon that when executed by a computer or aprocessing unit 32 perform certain steps. These steps can include,without being limited to, receiving the image acquired by the imagecapture device 30 of the imaging module 42 and, for each illuminatedparticle 22: identify the plurality of spots associated with theilluminated particle 22 in the plurality of sub-images; determine thespot parameter associated with each of the plurality of spots; anddetermine the size of the illuminated particle 22 from the plurality ofspot parameters.

As used herein, the term “computer readable memory” is intended to referto a non-transitory and tangible computer product that can store andcommunicate executable instructions for performing particle sizinganalysis from the image acquired by the image capture device. Thecomputer readable memory 43 can be any computer data storage device orassembly of such devices including, for example: a temporary storageunit such as a random-access memory (RAM) or dynamic RAM; a permanentstorage such as a hard disk; an optical storage device, such as a CD orDVD (rewritable or write once/read only); a flash memory; and/or othernon-transitory memory technologies. A plurality of such storage devicesmay be provided, as can be understood by one of ordinary skill in theart. The computer readable memory 43 may be associated with, coupled toor included in the processing unit 32, wherein the processing unit 32 isconfigured to execute instructions stored in the computer readablememory 43.

More details regarding various structural and operational features ofthe particle sizing system and the imaging module for use therein willnow be given below, with reference to the exemplary embodiments of FIGS.5 to 15B.

Referring more specifically to FIG. 5, there is shown a first exemplaryembodiment of a particle sizing system 20. The particle sizing system 20first includes an optical source 26. The optical source 26 generates alight beam 34 illuminating particles 22 contained in a monitored volume24, so as to cause part of the light beam 34 to be scattered by theilluminated particles 22. Each illuminated particle 22 producesscattered light 38 from a respective location in the monitored volume24. It will be recognized that while FIG. 5 depicts light 38 scatteredfrom only two particles 22 ₁, 22 ₂ for clarity, in practice, the numberof particles contained in the monitored volume 24 at any given timecould differ depending on the particular application.

The optical source 26 can be embodied by any appropriate device orcombination of devices apt to generate a light beam 34 suitable foroptical scattering-based particle sizing applications. By way ofexample, in some implementations, the optical source 26 may be a lasersource configured to generate the light beam 34 as a collimated laserbeam, for example with a relatively small divergence of the order of afew milliradians (e.g., between 1 and 3 milliradians). The laser beammay be monochromatic, although its spectrum could spread over a certainextent on either side of its central wavelength. The laser beam may bepolarized or not, and be operated in both continuous-wave and pulsedregimes.

By way of example, in a non-limiting embodiment, the optical source 26can be a frequency-doubled diode-pumped solid-state Nd:YAG laser modulegenerating a laser beam having a wavelength of 532 nm, a linewidthsmaller than 1 nm, a beam diameter smaller than 2.5 mm full width at1/e², an approximately Gaussian transverse irradiance profile, andcarrying an average power of approximately 10 milliwatts (mW). Ofcourse, various other types of laser sources appropriate to generate alaser beam having suitable characteristics may be used to perform thepresent techniques such as, for example, a gas laser, a solid-statelaser, a diode laser, a dye laser, a fiber laser, and the like. Thechoice of the optical source can be dictated by several factorsincluding, without limitation, the wavelength, power, spatial andspectral profiles, and, for a pulsed optical source, the pulsecharacteristics.

It will be understood that the irradiance of the light beam 34 isgenerally not uniform over its cross-sectional area, being usuallylarger in regions close to the optical axis 36 and to the optical source26. This means that the irradiance of the light incident on a particle22 will in generally vary with the particle location in the monitoredvolume 24. As a consequence, the intensity of scattered light will alsodepend on particle position within the monitored volume 24. In someimplementations, using a light beam 34 with a non-uniform irradiancedistribution, such as a Gaussian profile, may advantageously extend theparticle size range that can be measured with the particle sizing system20.

Turning briefly to FIGS. 15A and 15B, the light beam illuminating theparticles 22 present in the monitored volume 24 may be fan-shaped alongone of its transverse dimension. In such a case, the particle sizingsystem may further include a beam conditioning element 68 disposedbetween the optical source 26 and the monitored volume 24 for shaping orconverting the light beam 34 into a fan-shaped beam 70. As describedfurther below, using a fan-shaped beam 70 may provide a way to increasethe size of the monitored volume 24 (e.g., by one to three orders ofmagnitude), which may be advantageous at low particle concentrations.

The wavelength of the light beam 34 could be in the visible range tomatch most commercially available image capture devices, or in anyappropriate portion of the electromagnetic spectrum adapted to aparticular range of particle diameters to be measured. By way ofexample, a rule of thumb that may be utilized is that for angles ofobservation smaller than 25°, adequate particle sizing may be achievedif the light wavelength is about twice the particle diameter to bemeasured, as it would yield a signal ratio larger than 1.2 between thesmallest and largest observation angles. For example, for particleshaving a diameter larger than 250 nm, the difference in signal levelsbetween intensities scattered at 5° and 25° will generally be largerthan 20% with a wavelength lower than 500 nm.

Besides laser sources, other types of optical sources may be used insome embodiments including, without limitation, light-emitting diodes(LEDs) and other broadband light sources. By way of example, diode lasersources with a linewidth ranging from a few nm to about 10 nm orfiber-coupled LEDs with a linewidth in the range of 30 nm could be used.The beam radiated from a fiber-coupled LED could be collimated with asuitable beam collimator to get an illumination light beam with therequired or desired size characteristics. For example, this could allowthe light beam to change from a top-hat to a Gaussian transverseirradiance profile as the distance from the optical source increases. Insome implementations, the relatively flat cross-sectional irradianceprofile of top-hat beams could simplify the analysis of absolutescattered intensity measurement data and the calculation of particlesize distributions.

It is to be noted that the oscillations in the particle scatteringresponse as function of size and scattering angle could be reducedsignificantly when using LED sources, due to the averaging of thescattering cross section provided by the extended LED emission spectra.Accordingly, depending of the dimensions and wavelength ranges involvedthe Mie scattering theory may provide information on the extent of theseoscillations. If required, strategies for reducing the amplitude ofthese oscillations in the particle sizing analysis could then beimplemented.

Further embodiments can use optical sources having a broad spectralbandwidth, possibly in conjunction with a spectrally-resolving imagecapture device or with multiple image capture devices combined or notwith dichroic or Bayer optical filters. An alternative to a broadbandoptical source can involve using multiple optical sources generatingmonochromatic light beams of different colors and that are madecollinear. In yet other embodiments, the optical source could beembodied by a multi-frequency source (e.g., a Nd:YAG laser) withfrequency conversion capabilities for providing two or three outputwavelengths (e.g., 1064 nm, 532 nm and 355 nm). Such optical sources mayprovide complementary particle size distribution information byextending the range of particle sizes that can be measured, with theoptical scattering at shorter and longer wavelengths being generallymore sensitive to smaller and larger particles, respectively. It isnoted that using a multi-frequency or broadband optical source, incombination with an image capture device with spectrally-resolvedimaging capabilities could allow information on both particle size andcomposition to be obtained from the same system.

In the embodiment of FIG. 5, the particle sizing system 20 also includesa plurality of light deflectors 28 a to 28 c, each of which positionedto receive and deflect light 38 scattered from the illuminated particles22. As used herein, the term “light deflector” is intended to refer toan optical element or a combination of optical elements which canredirect, at least partly, the optical path of light incident thereonto.Each light deflector can be embodied by a reflecting, a refracting or adiffracting element, or a combination thereof. Non-limiting examples oflight deflectors include plane and curved mirrors, beam splitters,prisms, filters, diffraction gratings and holographic elements.

In the illustrated embodiment, three light deflectors 28 a to 28 c areprovided, although other embodiments may include two or more than threelight deflectors. In particular, a pair of light deflectors could besufficient used to achieve adequate particle sizing for a limited rangeof particle size. Depending on the application, the light deflectors maybe different from or identical to one another.

In the embodiment of FIG. 5, the plurality of light deflectors 28 a to28 c consists of three light reflectors embodied by plane mirrors. Theplane mirrors may have a reflectivity higher than 90% at the wavelengthof the light beam 34, although different values of reflectivity could beused in other embodiments. The plane mirrors can have identical ordifferent dimensions. For example, in FIG. 5, the first and second lightdeflectors 28 a, 28 b are square mirrors with sides 25 mm long, whilethe third light deflector 28 c is a square mirror with sides of 50 mm.These dimensions are provided for the purpose of illustration only, suchthat the size and shape of the light deflectors 28 a to 28 c may differin other embodiments. In the embodiment of FIG. 5, each of the lightdeflectors 28 a to 28 c is positioned relative to the intended monitoredvolume to intercept and deflect a respective forward scattered componentof the light 38 scattered from the illuminated particles 22, eachforward scattered component lying within a distinct scattering anglerange.

As mentioned above, in some embodiments, the particle sizing system 20may advantageously determine individual particle size based on ratios ofscattered intensities measured at different small forward scatteringangles in order to eliminate or at least mitigate the influence ofparticle composition on the size determination analysis. Additionally,in some cases, the plurality of light deflectors 28 a to 28 c may bepositioned so that their surface normals are parallel to a commonhorizontal plane, referred to herein as the “system plane”. In FIG. 5,the system plane is parallel to the plane of the page and contains boththe optical axis 36 of the light beam 34 and the optical axis of theimage capture device 30.

In order to use more efficiently the available photosensitive surface ofthe image capture device 30, the vertical dimension of each lightdeflector 28 a to 28 c may be selected to match the vertical dimensionof the field of view of the image capture device 30. Furthermore, thelight beam 34 illuminating the particles 22 could be shaped as afan-shaped beam, as depicted in FIGS. 15A and 15B, with the plane of thefan-shaped beam oriented perpendicularly to the system plane. In suchimplementations, a greater fraction, and possibly all, of thephotosensitive surface of the image capture device 30 could be used.

Referring still to FIG. 5, each light deflector 28 a to 28 c may becharacterized by an angle θ_(a), θ_(b), θ_(c) relative to the opticalaxis 36, a distance from the optical axis 36, and a distance from theimage capture device 30. The angle θ_(a), θ_(b), θ_(c) of each lightdeflector 28 a to 28 c is defined herein as the angle made between theoptical axis 36 of the illumination light beam 34 and a line extendingbetween a scattering point in the monitored volume 24 and the center ofthe deflector 28 a to 28 c, such that light scattered from thescattering point and impinging on the center of the deflector 28 a to 28c is deflected along a path that intersects the optical axis of theimage capture device 30 at the exit plane of the collecting optics 46.The distance of each light deflector 28 a to 28 c from the optical axis36 corresponds to the distance between the center of the light deflector28 a to 28 c and the location on the optical axis 36 of the vertex ofthe corresponding scattering angle θ_(a), θ_(b), θ_(c). Finally, thedistance of each light deflector 28 a to 28 c from the image capturedevice 30 corresponds to the distance between the center of the lightdeflector 28 a to 28 c and the point of intersection of the optical axisof the image capture device 30 on the exit plane of the collectingoptics 46.

In some implementations, additional light deflectors (not shown) couldbe added above and/or below the “main” light deflectors 28 a to 28 cillustrated in FIG. 5. Each of these additional light deflectors couldbe embodied by a plane mirror and be oriented slightly differentlyrelative to the monitored volume 24. While the surface normals to thedeflecting surfaces of the main light deflectors 28 a to 28 c areparallel to the system plane in FIG. 5, those of the deflecting surfacesof the additional light deflectors are not. The provision of theseadditional light deflectors could improve the accuracy of the particlesize determination, especially when the range of sizes of the particlescontained in the monitored volume 24 is wide. Also, by tilting theadditional light deflectors with respect to the main light deflectors 28a to 28 c, a larger proportion of the photosensitive surface of theimage capture device 30 could be used.

In the embodiment of FIG. 5, the particle sizing system 20 furtherincludes an image capture device 30 collecting deflected scattered light40 received within its field of view 44 from each of the lightdeflectors 28 a to 28 c. Advantageously, a single image capture devicecan be used to collect deflected scattered light from each lightdeflector. As used herein, the term “image capture device” refers to anydevice or combination of devices capable of acquiring an imagerepresenting light scattered by the illuminated particles in themonitored volume and containing information about the spatialdistribution of the illuminated particles in the monitored volume, suchthat particles at different locations in the monitored volume are imagedon distinct regions in the image. The term “field of view” refers to theangular extent of the scene that can be imaged by the image capturedevice. As mentioned above, in some embodiments, more than one imagecapture device may be provided if the optical source has a broadspectral bandwidth or if multiple optical sources emitting light indifferent spectral ranges are used. However, in such cases, each of theimage capture devices would generally be configured to collect deflectedscattered light from each light deflector.

The field of view 44 of the image capture device 30 encompasses at leastpartly, and in some cases entirely, each light deflector 28 a to 28 c.Additionally, it may be advantageous, in some implementations, that thefield of view 44 of the image capture device 30 be filled as much aspossible by the plurality of light deflectors 28 a to 28 c. Indeed,generally no valuable information relative to the particles 22 containedin the monitored volume 24 can be retrieved from regions of the imagethat correspond to dead zones between adjacent light deflectors.

In the embodiment of FIG. 5, the image capture device 30 can includecollecting optics 46 adapted to collect deflected scattered light 40from the plurality of light deflectors 28 a to 28 c. The collectingoptics 46 may include lenses, mirrors, filters, optical fibers and anyother suitable reflective, refractive and/or diffractive opticalcomponents. For example, in the illustrated embodiment, the collectingoptics 46 includes an objective. In a non-limiting exemplary embodiment,the image capture device 30 may have a field of view of 20°, with anobjective having a focal length of 12.5 mm and an f-number of 1.4, butother parameter values may be used in other embodiments.

The image capture device may also include a sensor array 48. The term“sensor array” refers herein to a device made up of a plurality ofphotosensitive elements (pixels) capable to detect electromagneticradiation incident thereonto from a scene, and to generate an image ofthe scene, typically by converting the detected radiation intoelectrical data. Depending on the application, the pixels may bearranged in a two-dimensional or a linear array. The term “pixel data”refers to the image information associated with each pixel and mayinclude intensity data indicative of the total amount of electromagneticenergy absorbed by the pixel over a certain period of time.

The sensor array 48 may be embodied by a complementarymetal-oxide-semiconductor (CMOS) or a charge-coupled device (CCD) imagesensor, but other types of sensor arrays (e.g., charge injection devicesor photodiode arrays) could alternatively be used. By way of example, ina non-limiting embodiment, the sensor array is embodied by CCD imagesensor made up of 640×480 pixels with 7.4 μm square pixels

Referring to FIG. 5 in conjunction with FIG. 16, the image capturedevice 30 is configured to output an image 50 consisting of a pluralityof sub-images 52 a to 52 c, each of which being generated from collectedlight deflected from a respective one of the plurality of lightdeflectors 28 a to 28 c. It will be understood that each illuminatedparticle 22 in the monitored volume 24 will be imaged as a spot 54(hereinafter referred to collectively and generally as 54, butindividually as 54 a ₁, 54 a ₂, 54 b ₁, 54 b ₂, 54 c ₁ and 54 c ₂) ineach one of the plurality of sub-images 52 a to 52 c. Therefore, theplurality of spots 54 associated with each particle 22 corresponds tolight scattered at a plurality of scattering angles.

By way of example, in FIG. 16, the image 50 includes three sub-images 52a to 52 c associated respectively with the three light deflectors 28 ato 28 c depicted in FIG. 5. Each sub-image 52 a to 52 c includes twospots 54, each spot 54 being associated with one of the two illuminatedparticles 22 ₁, 22 ₂ in the monitored volume 24. More specifically, thefirst sub-image 52 a includes a first spot 54 a ₁ representing lightscattered by the first particle 22 ₁ and deflected by the first lightdeflector 28 a, and a second spot 54 a 2 representing light scattered bythe second particle 22 ₂ and deflected by the first light deflector 28a. Meanwhile, the second sub-image 52 b includes a first spot 54 b 1representing light scattered by the first particle 22 ₁ and deflected bythe second light deflector 28 b, and a second spot 54 b ₂ representinglight scattered by the second particle 22 ₂ and deflected by the secondlight deflector 28 b. Finally, the third sub-image 52 c includes a firstspot 54 c ₁ representing light scattered by the first particle 22 ₁ anddeflected by the third light deflector 28 c, and a second spot 54 c ₂representing light scattered by the second particle 22 ₂ and deflectedby the third light deflector 28 c. It is noted that the size, shape andseparation of the spots 54 in FIG. 16 are not necessarily depicted toscale.

It will be understood that when the light beam 34 incident on theparticles 22 in the monitored volume 24 is a pencil beam and when thesurface normals to the deflecting surfaces of the plurality of lightdeflectors 28 a to 28 c are parallel to a common plane, then the spots54 acquired by the image capture device 30 will generally spread along aline in the image 50. However, in other configurations of the particlesizing system 20, spots 54 may be formed along two dimensions of theimage 50 without departing from the scope of the techniques describedherein.

It will be understood that one possible advantage of using a singleimage capture device is that the measurement of scattered light signalsreceived from different ones of the plurality of light deflectors can beautomatically synchronized through the exposure time of the imagecapture device. By contrast, using a plurality of independent imagecapture devices, each of which collecting scattered light from acorresponding one of the plurality of light deflectors may not bestraightforward. Indeed, depending on the time resolution of themeasurements and the transit time of the particles across the monitoredvolume, interpreting and analyzing the results can become quite complexwhen more than one image capture device is employed.

It will also be understood that the scattered path length between agiven particle and the image capture device will generally varydepending on which of the plurality of light deflectors has redirectedthe scattered light on the image capture device. Therefore, for eachparticle, the spots in different sub-images will generally not befocused with the same efficiency. Depending on the application, this mayor may not provide an advantage. By way of example, in the embodiment ofFIG. 5, the scattered path length is greater for scattered lightreaching the image capture device 30 via the third light deflector 28 c.Therefore, if the spot representing a given particle is sharply focusedin the third sub-image, it may be slightly blurred in the first andsecond sub-images. All things being equal, the measured signal wouldtend to be more intense for the smaller, more-focused spot in the thirdsub-image than for the larger, less-focused spot in each of the firstand second sub-images. However, this difference could be at least partlycompensated for by the fact that the scattering cross section efficiencywould tend to be smaller for the third light deflector 28 c. This is duenotably to the fact that the third light deflector 28 c deflects lightscattered at larger scattering angles and over a smaller solid anglethan the first and second light deflectors 28 a, 28 b. Therefore, takingadvantage of the different scattering cross section efficienciesassociated with the different light deflectors 28 a to 28 c can providea way to use the available dynamic range of the image capture device 30more efficiently.

It will be understood that particles are usually not stationary in themonitored volume since they are carried by the flow of air or liquid inthe host medium. By way of example, for a light beam having a diameterof about 4 mm and air velocity of about 1 meter per second (m/s) in adirection perpendicular to the optical axis 36, the transit time of aparticle across the width of the beam will be 4 milliseconds (ms). Theexposure time of commercially available image capture devices can be ofthe order of 1 ms and, in some cases, as low as 10 microseconds (μs). Inthe above example, using an exposure time longer than 4 ms with arefresh rate of 33 frames per second may increase the likelihood ofhaving a particle passing through the central region of the light beamduring the image acquisition process and, thus, the likelihood ofdetecting a particle. Hence, by increasing the exposure time, the sizeof the volume monitored by the particle sizing system is effectivelyincreased due to the fact that more particles cross the light beam.

The possibility to increase the effective size of the monitored volumeby increasing the exposure time may be useful when the particleconcentration is small (e.g., when there is on average one or less thanone particle present in the monitored volume during the exposure time).Furthermore, the signal intensity detected by the image capture devicefor a given particle will also increase if it represents the summationof a number of adjacent pixels above a threshold level, which reflectsthe fact that a moving particle may produce a linear trace in the imageacquired by the image capture device.

It is also worth mentioning that by increasing the exposure time, thelikelihood that a given particle passes through a high-intensity regionof the light beam also generally increases, thus making the particlemore readily detectable. However, at high particle concentrations,increasing the exposure time may increase the likelihood of coincidentalparticle detections which, in turn, may affect the size determinationanalysis. It may further be possible to use different exposure timesover a certain period of time, for example over one minute or onesecond.

The rate at which images are acquired by the image capture device mayalso affect the monitored volume over time. By way of example, in someimplementations, the refresh rate of the image capture device may beselected so as to avoid a certain particle to be detected in one regionof the light beam in one image, and in another region of the light beamin a subsequent image, as this could artificially increase the count andconcentration level for the corresponding particle size, especially forlarger particles that may still be detected close to the edge of themonitored volume.

It will thus be understood that, depending on the particle concentrationlevel, compromises may need to be made between the exposure time, therefresh rate, the aperture size and the depth of focus of the imagecapture device in order to provide accurate particle sizing capabilitiesover different particle size ranges and accurate statistical resultsfrom which particle size distribution information may be determined.

In some implementations, it may be advantageous to reduce or at leastmanage the ambient light conditions in order to operate the particlesizing system in different environmental conditions. Ambient light cangenerally manifest itself in the image acquired by the image capturedevice in two ways. First, ambient light can illuminate backgroundobjects present in the field of view of the image capture device, which,in turn, can generate a signal in the image. Second, ambient light canilluminate the particles in the monitored volume, thereby generatingscattered light that adds to the scattered light caused by theillumination light beam.

Therefore, in some implementations, it may desirable or required toreduce ambient light contamination, for example with temporal filtering(e.g., by using a pulsed or modulated optical source combined with asynchronous detection scheme) or spatial filtering (e.g., by usingdedicated deflectors or baffles around the monitored volume and/or nearthe optical source). In particular, in a non-limiting exemplaryembodiment, the power of the optical source may be modulated or pulsedsynchronously with the image capture device. In such a case, if at eachmodulation or pulse the power output is changed, an extended equivalentdynamic range may be achieved from the combinations of different opticalsource powers at different times, assuming that the particle sizedistribution does not vary much over the time period of the powermodulation or pulse. In such implementations, it may be easier to takeinto account the manner in which the probability of detecting particlesand the size of the effective monitored volume each depend on particlesize, thus possibly optimizing the overall statistics for differentclasses of particle size.

In the embodiment of FIG. 5, the particle sizing system 20 also includesa processing unit 32 that retrieves the image acquired by the imagecapture device 30 from a computer readable memory 43 included in theprocessing unit 32. In such a case, the processing unit 32 is configuredto execute instructions stored in the computer readable memory 43. Insome embodiments, the processing unit 32 and the image capture device 30may be integrated as a single unit. In other embodiments, the processingunit 32 and the image capture device 30 may be distinctly separate. Insuch a case, the processing unit 32 may be operatively connected to theimage capture device 30 via wired or wireless communication links. Insome embodiments, the processing unit 32 may also be connected to othercomponents of the particle sizing system 20, for example the opticalsource 26, as depicted in FIG. 5.

Depending on the application, the processing unit 32 can start toanalyze the image upon receiving it from the image capture device 30(i.e., in real-time or near real-time), or may store the image for lateranalysis.

Referring again to both FIGS. 5 and 16, the processing unit 32 isconfigured to identify the plurality of spots 54 associated with eachparticle 22 in the plurality of sub-images 52 a to 52 c. It is to benoted that with appropriate mapping between spatial and imagecoordinates, it may be possible to determine on which pixels of eachsub-image 52 a to 52 c is recorded the light scattered from a particle22 at a given location in the monitored volume 24 after deflection froma given one of the light deflectors 28 a to 28 c. The different spots 54associated with a given illuminated particle 22 can thus provide ameasure of the distribution of the intensity of light scattered from theparticle 22 at different scattering angles.

In other words, the spot identification process can make it possible toassign both an angle and a distance of observation for each spot 54 inthe image 50 acquired by the image capture device 30. Using this mappingbetween spot locations in the image 50 and particle locations in themonitored volume 24, differences in signal strength at differentobservation angles can be accounted for by considering the differencesin the distances of observation associated with different lightdeflectors 28 a to 28 c. In some implementations, the identification ofspots 54 associated with each particle can involve searching andidentifying, in the region of the image 50 corresponding to themonitored volume 24, pixel signal levels that are above certainthresholds. The thresholds may have been previously established throughcalibration. The spots may be defined by grouping adjacent pixels whosesignal levels are above one of the predetermined thresholds.

It is noted that using a plurality of light deflectors to form an imagemade of a corresponding plurality of sub-images, each of whichrepresenting a different angle of observation of the monitored volume,is similar to using multiple image capture devices to obtain informationabout the monitored volume from different points of view. Such atechnique is also known as stereoscopic imaging. As known in the art,stereoscopic imaging can allow the position of an object in athree-dimensional space to be assessed by identifying and correlatingfeatures of the object as observed from different vantage points.

With proper calibration of the positions of the light deflectors and theimage capture device relative to one another and to the light beam, thepresent techniques can make use of principles similar to those used instereoscopic imaging to map spot locations in the image acquired by theimage capture device to particle locations in the monitored volume. Thismapping can allow the determination of the angle and distance of eachparticle relative to the image capture device and the plurality of lightdeflectors. This positional information can be used to properly comparemeasurement and reference data. Furthermore, depending on the resolutionof the image capture device, information on particle location may beobtained not only along the optical axis of the light beam, but alsowithin its cross section.

In some implementations, a more accurate determination of the particlesize distribution may be obtained when information about the transverseirradiance profile of the light beam and the position of particleswithin its cross section is used. Particle classification based oncomparing the magnitude of scattered intensity signals will generallydepend on the position of the particles within the cross-sectional areaof the illuminating light beam. As mentioned above, the irradiance isgenerally not uniform over the cross-sectional area of the light beam,being generally larger at the center of the beam and progressivelydecreasing toward its periphery. The scattered light intensity beingdependent on the incident light intensity, it will follow the samebehavior.

The processing unit 32 is also configured to compute a spot parameterfor each of the plurality of spots 54 associated to a given particle 22.As mentioned above, in some implementations, the spot parameters may beenergy parameters, each of which being indicative of an amount of energycontained in the spot 52 associated therewith. In other implementations,the spot parameters may correspond to a size, a shape, a spectralcontent of the spots, or any combination thereof. Determining the spotparameter of a particular spot may involve summing the pixel signals ofall the pixels belonging to this spot. In some implementations, thesignal intensity of each spot may be corrected by taking into accountthe distance of observation of the scattering particle associated withthe spot.

The processing unit 32 is further configured to compute a size of eachone of the illuminated particle 22 from the plurality of spot parametersassociated therewith. In some implementations, the processing unit 32can be configured to determine the size of each illuminated particle 22from one or more ratios between the spot parameters (e.g., energyparameters). By way of example, in an embodiment where the scatteringangles of the scattered light collected by the image capture device 30after deflection from the plurality of light deflectors are respectivelyθ_(a), θ_(b), θ_(c), one or more of the following ratios may becalculated: R₁(θ_(a), θ_(b))=I_(θa)/I_(θb), R₂(θ_(a),θ_(c))=I_(θa)/I_(θc) and R₃(θ_(b), θ_(c))=I_(θb)/I_(θc). In theseratios, the quantities I_(θa), I_(θb), and I_(θc) correspond to theintensities of collected light scattered from an illuminated particle 22at a scattering angle equal to θ_(a), θ_(b), θ_(c).

The processing unit 32 may be further configured to determine the sizeof the particle 22 associated with each set of spots 54 from acomparison of the one or more of intensity ratios R₁(θ_(a), θ_(b)),R₂(θ_(a), θ_(c)) and R₃(θ_(b), θ_(c)) with reference data. It will beunderstood that while a certain number of intensity ratios may becalculated depending on the number of light deflectors 28 a to 28 c, notall these ratios need to be used in every calculation if using a lowernumber of ratios allows a sufficiently accurate particle sizedetermination.

Also, as mentioned above, the reference data can be obtained from amodel based on the Mie scattering theory or another suitable theory. Byway of example, the Mie theory may be used to calculate, for eachparticle identified in the image, theoretical values for the ratiosR₁(θ_(a), θ_(b)), R₂(θ_(a), θ_(c)) and R₃(θ_(b), θ_(c)) for a range ofparticle sizes, and to find the particle size for which these ratiosbest fit those obtained from the measurement data. It will be understoodthat given the many computational approaches available for numericallyor analytically modeling the scattering response of particles, varioustechniques could be employed to obtain the reference data. Alternativelyor additionally, at least part of the reference data may originate frompreviously calculated and/or measured experimental data stored in adatabase. Such a database may include reference data for severalparticle sizes, compositions, shapes, distances, scattering angles, andthe like. Depending on the application, the step of obtaining referencedata and the step of comparing the reference data with measurement datamay each be carried out in real-time or near real-time, orretrospectively as post-measurement steps.

In some implementations, the Mie scattering theory can be used tocalculate the angular scattering cross section of a particle at thecorresponding wavelength(s) of the illumination light beam and for arange of scattering angles and a range of particle sizes. Thecalculation may also be performed for different refractive indicescorresponding to different particle compositions. By way of example, inone non-limiting embodiment, three different refractive indices may bechosen, corresponding to quartz, dust-like and coal particles. Lightpolarization may also be taken into account, if required or desired.Ratios of scattered intensities measured at different observation anglesand associated with spots belonging to a same particle may then becalculated, taking into account the angular coverage of the particlesizing system at each observation angle. Each set of spots associatedwith a particle should include at least two spots in order for at leastone ratio to be calculated therefrom. Optionally, the spectral contentof the light beam illuminating the particle may also be accounted for.Finally, the particle size at which the calculated ratios best match themeasured ratios is found. In some implementations, criteria includingsignal levels and ratios possibly calculated from extrapolated saturatedsignal can be taken into account in the assessment of the best match.This process may be repeated for each set of spots.

As mentioned above, the scattered path length between a given particleand the image capture device will generally vary depending on which ofthe plurality of light deflectors has redirected the scattered lightonto the image capture device. In some implementations, these differentscattered path lengths may significantly affect the measured intensityof scattered light received from the different light deflectors, andneed to be taken in account when ratios between scattered intensitiesmeasured at pairs of scattering angles are compared with reference data.

It will also be understood that, in general, the scattering angleassociated with a given spot in the image of the monitored volumeacquired by the image capture device covers a certain range ofscattering angles (e.g., less than 1°) around a main value. Thisscattering angle range can vary as a function of the location of thescattering particle in the monitored volume and depending on which ofthe light deflectors is involved in the collection process. The span ofthe different scattering angle ranges may affect the intensity of themeasured scattered signals and may have to be accounted for whencomparing measurement and reference data.

It is worth mentioning that, while some implementations may use ratiosor other relations between scattered intensities at different scatteringangles, other implementations may alternatively or additionally useabsolute values of scattered intensities. As mentioned above, absolutemeasurements may be useful to pre-classify each particle as belonging toone of different particle size ranges. By way of example, the absolutevalues of scattered intensities at different angles may be used topre-categorize particles in the monitored volume as being small (e.g.,with a diameter smaller than 1 μm), intermediate (e.g. with a diameterbetween 1 μm and 10 μm) and large (e.g., with a diameter larger than 10μm). It will be understood that particle sizing using absolutemeasurements may be more sensitive to the accuracy with which theparticle position can be determined than particle sizing based solely onrelative measurements.

As mentioned above, the optical source may be embodied by amulti-wavelength or a broadband source while the image capture devicemay have spectral imaging capabilities. In such implementations, thespots identified in the image acquired by the image capture device maycontain information about the spectral content of the scattered lightcollected at different scattering angles. In turn, this spectral contentmay be used to obtain information about both the size and composition ofthe particle. In some implementations, the polarization of the collectedscattered light may be measured in order to yield information about thesize of smaller particles (e.g., less than 200 nm in diameter), theshape, the phase (e.g., solid or liquid) and/or the composition of theparticles.

In some implementations, the particle sizing system can advantageouslyallow the detection of particles that are at different locations insidethe monitored volume. Therefore, in some implementations, the monitoredvolume could be made larger with the present techniques than withconventional techniques in which particles are supplied to the opticalinterrogation region using a vacuum-based pumping system.

Another advantage of some embodiments of the present techniques is thatthey can allow the size of more than one particle to be determined permeasurement or image. This is typically not possible with conventionallight scattering particle counters, since they are usually limited interms of the particle concentration level than can be measured withouthaving interference between particles passing simultaneously in theoptical detection area. By contrast, some of the techniques describedherein can provide different sets of scattered signals for particlesthat are at different locations in the monitored volume, unless theparticles are superposed or stuck together.

The present techniques may also be advantageous at low concentrationlevels by providing a statistically representative particle sizedistribution in a shorter period of time due to the relatively largemonitored volume that can be achieved. In some implementations, aparticle size distribution may be obtained by combining individualparticle size measurements acquired either sequentially over themonitored volume or simultaneously for multiple particles inside themonitored volume if the monitored volume is sufficiently large.

Another possible advantage of some implementations of the techniquesdescribed herein is that they may allow non-intrusive particle sizemeasurements to be performed in situ and from a standoff position. Inparticular, in some implementations, particle sizing may be achieved inan open air environment and without requiring the particles to besampled from the ambient medium and supplied to the monitored volume bya vacuum-based pumping system. In situ measurements can prevent or atleast reduce environmental perturbations, interferences and bias causedby the particle sampling process and/or the presence of the particlesizing system itself and any associated components (e.g., inlets biasedto a certain particle size, particles breaking up as a result of hittingsystem components, particle deposition on wall surfaces, and the like).Additionally, with in situ measurements, there is generally no need tocalibrate and maintain pumping equipment to ensure proper knowledge ofthe supply rate of particles in the monitored volume. Of course, it willbe understood that, in some implementations, a pumping system may beused without departing from the scope of the present techniques.

The techniques described herein allow the size of individual particlesin a monitored volume to be determined based on the assumption thatscattered light from different particles correspond to different spotsin the image acquired by the image capture device. In other words, it isassumed that the particle sizing system will detect only one particle ata time at a given location in the monitored volume. In someimplementations, in order for this assumption to be fulfilled, theaverage number of particles within a certain region of space should notexceed one. In a first approximation, this means that the averagedistance between particles in the monitored volume should be sufficientto produce distinct spots in the image acquired by the image capturedevice. In other words, the light signals scattered from differentparticles should preferably not overlap on the image. Otherwise, it maybecome more challenging to determine particle size with sufficientaccuracy.

In the techniques described herein, the number of particles detectedwithin a portion of the light beam can be generally less than one inmost environmental and occupation health and safety applications. Also,when the particle concentration increases to a value that causes thenumber of particles to be detected at a certain observation angle tobecome larger than one, it may be possible to reduce the field of viewor increase the resolution of the image capture device in such a way asto reduce the average size of the portion of the monitored volume imagedon any single pixel.

Referring to the embodiment of FIGS. 6A to 6C, by way of example, animage capture device 30 with 640 (H)×480 (V) pixels and a field of viewof 20° will be characterized by an angular resolution of 0.03125° perpixel. Each light deflector 28 a to 28 c may be positioned to coverabout 6.3° of the field of view of the image capture device 30. Such aconfiguration would allow scattered light to be measured at threedifferent observation angles θ_(a), θ_(b), θ_(c) for each particle 22present in the monitored volume 24.

Referring to FIG. 6A, assuming that the minimum scattering angleθ_(a,min) associated with the first light deflector 28 a is equal to 1°,then the maximum scattering angle θ_(a,max) would be equal to 7.3°. Itis understood that scattered light collected at different scatteringangles in the range from θ_(a,min) to θ_(a,max) would originate fromdifferent locations in an interval 64 a along the optical axis 36 of thelight beam 34, the location associated with the scattering angleθ_(a,max) being closer to the first light deflector 28 a than thelocation associated with the scattering angle θ_(a,min).

FIG. 6B depicts the interval 64 b along the optical axis 36 covered bythe second light deflector 28 b between the minimum and maximumscattering angles θ_(b,min), θ_(b,max), while FIG. 6C depicts theinterval 64 c along the optical axis 36 covered by the third lightdeflector 28 c between the minimum and maximum scattering anglesθ_(c,min), θ_(c,max). While each of the three light deflectors 28 a to28 c is positioned to cover about 6.3° of the field of view of the imagecapture device 30, the lengths of the intervals 64 a to 64 c within theangular coverage of each deflector 28 a to 28 c differ substantially.

It is noted that the monitored volume 24 of the particle sizing system20 corresponds to the overlap of the three intervals 64 a to 64 c suchthat an illuminated particle 22 in the monitored volume 24 will formthree different spots in the image acquired by the image capture device30, each spot corresponding to a different angle of observation and adifferent scattered path length. Then, as mentioned above, threedifferent ratios R₁=I_(θa)/I_(θb), R₂=I_(θa)/I_(θc) and R₃=I_(θb)/I_(θc)may be calculated from the intensities I_(θa), I_(θb), and I_(θc) of thelight scattered from the particle 22 and measured at three differentobservation angles θ_(a), θ_(b), θ_(c) after deflection from the threelight deflectors 28 a to 28 c, respectively.

As illustrated in FIGS. 6A to 6C, in some implementations, each lightdeflector 28 a to 28 c can allow the image capture device 30 to view arelatively large portion of the light beam 34 along the optical axis 36and to collect light scattered by each illuminated particle present inthe monitored volume from multiple angles of observation. It will beunderstood that having a relatively large monitored volume 24 whileensuring that at most one particle 22 at a time is present on a smallportion of the light beam 34 can allow multiple particles to be imagedon different regions of the image acquired by the image capture device30. Referring also to FIG. 16, it may then become possible to associatethe location of each spot 54 in the image 50 whose intensity is above acertain threshold with a corresponding particle location in themonitored volume 24 and a corresponding observation angle θ_(a), θ_(b),θ_(c) associated with one of the light deflectors 28 a to 28 c.

In some implementations, having particles flowing through the monitoredvolume 24 rather than stationary particles can make the particle sizingsystem 20 less susceptible to have an impact on the particle motion and,therefore, on the particle size distribution. Particle movements mayalso ensure or help ensure that particles are not counted more thanonce. Generally, this is true as long as the time between successiveimages acquired by the image capture device 30 remains longer than thetransit time of the particles across the light beam. By way of example,assuming that the diameter of the light beam 34 is 0.5 cm and theparticles flow perpendicularly to the optical axis 36 at a speed of at20 cm/s, then the particle population in the monitored volume will berefreshed between successive images as long as the time betweenacquisitions of successive images is longer than 25 ms. If the particlespeed is increased to 2 m/s, then the time between successive images canbe reduced to 2.5 ms.

Referring now to FIG. 7, another embodiment of a particle sizing system20 is shown. This embodiment shares many features with the embodimentdescribed above with reference to FIG. 5 in that it generally includesan optical source 26, a plurality of light deflectors 28 a to 28 c, animage capture device 30, and a processing unit 32. In the embodiment ofFIG. 7, the particle sizing system 20 further includes a housing 56enclosing at least the plurality of deflectors 28 a to 28 c and theimage capture device 30. In some implementations, either or both of theoptical source 26 and the processing unit 32 may also be provided insidethe housing 56 to facilitate deployment of the particle sizing system 20in the field.

As used herein, the term “housing” refers to an enclosure that defines aspace for accommodating therein at least the plurality of deflectors andthe image capture device of the particle sizing system. The housing 56may be formed as a single integral structure or from two or more housingsections connected to form the housing. In some embodiments, the housing56 can prevent or help to prevent foreign matter such as rain, snow,mist, fog, dust, pollen and the like from reaching the light deflectors28 a to 28 c and the image capture device 30 during field deployments,for example in environmental or industrial monitoring applications,particularly in open air conditions.

The housing 56 can also reduce the risks of damaging or causingmisalignment of the light deflectors 28 a to 28 c, the image capturedevice 30 or other components as a result of accidental shock to orinadvertent mishandling of the particle sizing system 20. In someimplementations, the housing 56 may be smoothly shaped, for example withrounded edges, to minimize or at least reduce air flow interference andturbulence, and thus favor an unimpeded air flow around it. In someimplementations, the housing 56 may be portable.

In some implementations, the imaging module 42 of FIG. 7 may bemanufactured and sold as a single unit for use with the optical source26 and the processing unit 32 to form the particle sizing system 20. Insuch a case, the imaging module 42 may include the housing 56 providedwith the optical window 62 and enclosing the plurality of lightdeflectors 28 a to 28 c and the image capture device 30. The imagingmodule 42 could further be sold in combination with or in a kitincluding a computer readable memory 43 configured to be coupled to theprocessing unit 32 in such way as to allow the processing unit 32 toexecute instructions stored in the computer readable memory 43. Thecomputer readable memory 43 could then be embodied by a non-transitorystorage device such as, for example, a hard disk, a CD, a DVD or a flashmemory, while the processing unit 32 could be embodied by a personalcomputer.

In the embodiment of FIG. 7, the optical source 26 is provided outsideof the housing 56 and positioned at a standoff distance from themonitored volume 24. However, in other embodiments, the optical source26 may be located inside the housing 56. When the optical source 26 isoutside of the housing 56, the light beam 34 generated by the opticalsource 26 may be aligned with respect to the housing 56 by usingalignment pinholes 58 a, 58 b provided on the housing 56. Alternatively,one or both of the alignment pinholes 58 a, 58 b could be replaced byposition sensitive detectors. A beam dump 60 may also be provideddownstream the alignment pinholes 58 a, 58 b for better safety. When theparticle sizing system 20 is used in the field, the housing 56 and theoptical source 26 may each be mounted on tripods or other types ofmounting devices (not shown) to provide stable positioning. In someimplementations, the optical source 26 may be installed on an adjustablemounting device (not shown) to facilitate the alignment of the lightbeam 34 relative to the pinholes 58 a, 58 b provided on the housing 56.

Referring still to FIG. 7, the housing 56 can further include an opticalwindow 62 for allowing part of the light 38 scattered from the particles22 to be transmitted inside the housing 56 and reach the plurality oflight deflectors 28 a to 28 c which, in turn, will deflect the scatteredlight 38 toward the image capture device 30. It will be understood thatthe optical window 62 may be embodied by an opening or aperture definedthrough the housing 56 or by appropriate optics (e.g., a glass plate)configured to transmit light at the wavelength of interest. In someimplementations, engineering measures may be set up to avoid dust orother foreign matter to accumulate on the outer surface of the opticalwindow 62, for example by applying a dedicated coating on it orproviding an air curtain in front of it. In some implementations, theparticle sizing system 20 may not be significantly affected by foreignmatter accumulation on the optical window 62 as long as the opticalwindow 62 can transmit a sufficiently large fraction of the scatteredlight 38 incident thereon. This may be particularly true inimplementations where the particles 22 in the monitored volume 24produce a sharp focused image on the image capture device 30 while theimages of the dust particles present on the optical window 62 arecompletely out of focus.

In some implementations, the optical window 62 may be embodied by anaperture. In such a case, the inside of the housing 56 may be positivelypressurized relative to the outside of the housing 56 and, thus, to themonitored volume 24. In such cases, the maintenance of a positivepressure inside the housing 56 may ensure or help ensure that air iscontinuously coming out of the housing 56 through the aperture toprevent or help prevent airborne matter from reaching the lightdeflectors 28 a to 28 c and the objective of the image capture device30.

Turning now to FIG. 8, in some implementations, the same optical source26 could be used with two distinct imaging modules 42′, 42″, thusforming two particle sizing systems 20′, 20″. In FIG. 8, each imagingmodule 42′, 42″ includes a housing 56′, 56″ accommodating therein aplurality of light deflectors 28 a′ to 28 c′, 28 a″ to 28 c″ and animage capture device 30′, 30″. The two particle sizing systems 20′, 20″could be configured to interrogate substantially the same monitoredvolume 24 using different arrangements for the light deflectors 28 a′ to28 c′, 28 a″ to 28 c″ and the image capture device 30′, 30″. As aresult, the two particle sizing systems 20′, 20″ can measure scatteredintensities at different scattering angles to provide different yetcomplementary particle size information. In FIG. 8, the two particlesizing systems 20′, 20″ use the same processing unit 32, althoughdifferent processing units could alternatively be used. In some cases,the housings 56′, 56″ of the two imaging modules 42′, 42″ may beoriented with respect to each other in a manner such that the twoparticle sizing systems 20′, 20″ are configured to detect scatteredlight in two different system planes, for example in two perpendicularsystem planes. Such a configuration could provide information relativeto the shape of the particles 22 in the monitored volume 24.

An advantageous aspect of some embodiments of the present techniques isthat the dimensions of the particle sizing system may be scalable. Forexample, implementations such as depicted in FIGS. 7 and 8 could bedeployed in the field for environmental or industrial process emissionapplications where the light deflectors can be separated from themonitored volume by distances ranging from about a few decimeters to afew meters.

Referring now FIG. 9, in other implementations, the particle sizingsystem 20 may be scaled for integration into personal protectiveequipment such as helmets and safety glasses. In such a case, thecharacteristic size of and separation between the components of thesystem 20 can become of the order of a few centimeters. It is alsocontemplated that such a scaled-down version of the particle sizingsystem could be implemented on a mobile device equipped with a camera,such as a cell phone, a smartphone or a tablet computer. In suchimplementations, the image capture device could be embodied by thecamera of the mobile device itself. Furthermore, an application orsystem software could be provided on the mobile device to control andretrieve the images acquired by the camera and to process, either on themobile device itself or remotely through web- or cloud-based means, themeasurement data to obtain particle size information.

Referring now to FIGS. 10 to 14B, other embodiments of a particle sizingsystem 20 are shown. Again, these embodiments share many features withthe embodiment described above with reference to FIG. 5 in that theygenerally include an optical source 26, a plurality of light deflectors28 a to 28 c, an image capture device 30, and a processing unit 32.However, the embodiments of FIGS. 10 to 14B differ from the embodimentof FIG. 5 by the positional configuration used for light deflectors 28 ato 28 c and the image capture device 30. Each configuration may havedifferent advantages in terms of the relative dynamic range and theangles of observation.

Referring first to FIG. 10, the illustrated embodiment of the particlesizing system 20 depicts that by properly changing both the location andthe size of one of the light deflectors 28 a to 28 c (i.e., the thirdlight deflector 28 c in FIG. 10), the path length and the observationangle of the scattered light collected by the image capture device 30can be changed while maintaining the same angular coverage in the fieldof view 44 of the image capture device 30.

Referring now to FIG. 11, there is illustrated another embodiment of theparticle sizing system 20. In this embodiment, the locations of theplurality of light deflectors 28 a to 28 c are the same as in FIG. 5,but their orientations as well as the position of the image capturedevice 30 are different. First, it is seen that the image capture device30 and the plurality of light deflectors 28 a to 28 c are all located onthe same side of the light beam 34. This means that each one of theplurality of light deflectors 28 a to 28 c is positioned to deflectlight scattered from the particles 22 away from the optical axis 36(i.e., the propagation direction of the light beam 34). In other words,the scattered light deflected by each of the light deflectors 28 a to 28c reaches the image capture device 30 without crossing the light beam34. It will be understood that positioning the image capture device 30on the same side as the light deflectors 28 a to 28 c reduces thelikelihood of interference between deflected scattered light (i.e.,scattered light collected by the image capture device 30 afterdeflection from one of the light deflectors 28 a to 28 c) and directscattered light (i.e., scattered light directly collected by the imagecapture device 30, without prior deflection from one of the lightdeflectors 28 a to 28 c).

Second, the distance between the third light deflector 28 c and theimage capture device 30 in FIG. 11 is shorter when compared to theembodiment illustrated in FIG. 5. If the deflected scattered light fromthe third light deflector 28 c is also received in focus on the imagecapture device 30 while the deflected scattered light from each of thefirst and second light deflectors 28 a, 28 b is received slightlyoff-focus, then the intensity of the signal measured at the observationangle associated with the third light deflector will be enhanced.

Referring to FIG. 12, there is illustrated another embodiment of theparticle sizing system 20. In this embodiment, the enhancement of theintensity of the scattered light received from the third light deflector28 c could be even greater than in the embodiment of FIG. 11, due to theshorter distance between the third deflector 28 c and the image capturedevice 30.

Referring to FIG. 13, there is illustrated another embodiment of theparticle sizing system 20 in which the plurality of light deflectors 28a to 28 c are embodied by concave mirrors rather than plane mirrors.Concave mirrors not only deflect but also focus at least partly thelight 38 scattered from the illuminated particles 22 toward the imagecapture device 30. As a result, a larger proportion of the deflectedscattered light 40 can be collected, thus increasing the signal level onthe corresponding pixels of the image. It will also be understood thatthe mirror curvatures may be selected such that the deflected scatteredlight is collected in focus for each of the mirrors. In suchimplementations, the image capture device 30 may not have to be providedwith its own imaging optics (e.g., an objective lens). It will beunderstood that other embodiments of the particle sizing system 20 couldalternatively or additionally use convex mirrors as light deflectors.

Another advantage of the embodiment of FIG. 13 is that the angularcontent of each spot in the image acquired by the image capture device30 represents a larger range of scattering angles. As mentioned above,each spot represents light scattered from a given particle 22 collectedafter deflection from a given one of the light deflectors 28 a to 28 c.As a result, the oscillations in the angular scattering cross sectionfor larger particles can be reduced, which can facilitate the analysisof the scattering measurement data.

However, using concave rather than plane mirrors generally reduces thesize of the monitored volume 24 in each image acquired by the imagecapture device 30. At the same time, the smaller monitored volume 24 isaccompanied by an increase in the spatial resolution achievable with theimage capture device 30, and thus, by a decrease in the minimumseparation below which adjacent particles cannot be distinguished in theimages acquired by the image capture device 30. Therefore, using concavemirrors may be advantageous in the case of large particle concentrationlevels.

Referring now to FIGS. 14A and 14B, another embodiment of the particlesizing system 20 is illustrated that includes two additional lightdeflectors 28 d, 28 e to collect light 66 backward scattered from theparticles 22. In the illustrated embodiment, the fourth light deflector28 d is a concave mirror and the fifth light deflector 28 e is a convexmirror, but other arrangements could be used in other embodiments. Thearrangement shown in FIGS. 14A and 14B can ensure or help ensure thatthe backward scattered light 66 is properly focused on the image capturedevice 30 for a certain range of scattering angles. In this regard, inthe embodiment of FIGS. 14A and 14B, the optical path length of thebackward scattered light may significantly exceed the optical pathlength of forward scattered light. Therefore, if flat rather than curvedmirrors were used to redirect backward scattered light onto the imagecapture device 30, the resulting difference in the scattered intensityof backward and forward scattered light could become large enough toprevent or at least complicate particle size determination, especiallyconsidering that the intensity of backward scattered light at a givendistance is generally much smaller than the intensity of forwardscattered light for particles larger than the light wavelength.

It is also worth mentioning that, in some implementations, valuableinformation about the composition or refractive index of particles maybe retrieved from ratios of forward and backward scattered light. By wayof example, combining information on particle composition withinformation on particle size obtained from scattered light received fromthe first three light deflectors 28 a to 28 c can provide a mean todiscriminate individual particles having different compositions, in somecase even for particles mixed in an aerosol cloud. In such embodiments,particle size distributions could be determined independently fordifferent particle compositions. By way of example, it is known thatdust particles in a smoggy environment may change composition fromchemical reactions with smog gases such as, for example, ozone, NO_(x)and SO_(x). In such a case, embodiments of the particle sizing systemsuch as that illustrated in FIGS. 14A and 14B could allow the effect ofsuch chemical reactions on particle composition to be assessed in realtime and/or for different particle sizes.

Particle Sizing Method

According to another aspect, there is provided a particle sizing method.FIG. 17 depicts a flow chart of an embodiment of the method 100, whichcould, by way of example, be performed with a particle sizing system asdescribed above with reference to the embodiments of FIGS. 5 to 15B, orwith another particle sizing system.

The method 100 first includes a step 102 of illuminating particlescontained in a monitored volume so as to cause a part of the lightincident on the particles to be scattered.

The method 100 also includes a step 104 of receiving and deflectinglight scattered by the illuminated particles with a plurality of lightdeflectors. The light deflectors may be embodied by reflecting orrefracting optical elements such as, for example, plane and curvedmirrors, beam splitters, and prisms. More specifically, each lightdeflector may be positioned such as to deflect a respective component ofthe light scattered from each particle in the monitored volume,corresponding to a respective range of scattering angle. In someimplementations, the light scattered by the illuminated particles thatis received and deflected with the plurality of light deflectors may beforward scattered light.

The method 100 further includes a step 106 of collecting and imagingdeflected scattered light from each light deflector with an imagecapture device. Advantageously, in some implementations, a single imagecapture device can be used to collect deflected scattered light fromeach and every one of the plurality of light deflectors.

The method 100 next includes a step 108 of outputting an image generatedby the image capture device from the deflected scattered light collectedthereby. The image includes a plurality of sub-images, where eachsub-image is generated from the collected light deflected from arespective one of the plurality of light deflectors. Each illuminatedparticle is imaged as a spot in each sub-image, such that the pluralityof spots associated with a given particle corresponds to light scatteredby this particle at a plurality of different scattering angles.

The method 100 also includes a step 110 of identifying the plurality ofspots associated with each illuminated particle in the plurality ofsub-images. In particular, with proper knowledge or calibration of therelative positions of the light beam illuminating the particles, thelight deflectors and the image capture device acquiring the image of themonitored volume, it may be possible to associate a location in themonitored volume with each spot in each sub-image. In other words, itmay be possible to determine where the light scattered by eachilluminated particle at a particular location in the monitored volumewill form a spot in each of the sub-images after deflection from thecorresponding one of the light deflectors.

The method 100 next includes a step 112 of determining, for eachilluminated particle, a spot parameter associated with each spot of theplurality of spots, followed by a step 114 of determining a size of eachilluminated particle from the plurality of spot parameters associatedtherewith. In some implementations, the step 112 of determining a spotparameter associated with each of the spots may include determining anenergy parameter indicative of an amount of optical energy contained inthe spot associated therewith. In other implementations, the spotparameters may correspond to a size, a shape, a polarization or aspectral content of the spots, or any combination thereof.

In some implementations, the step 114 of determining the size of eachilluminated particle can include calculating or otherwise obtaining oneor more ratios of the spot parameters associated with each illuminatedparticle, and comparing the one or more ratios with reference data. Byway of example, the reference data can be obtained from a numerical oranalytical model based on the Mie scattering theory or another suitabletheoretical framework allowing a reference scattering response of theparticles to be obtained. In particular, the manner of obtaining thereference data is not meant to limit the scope of application of thepresent method 100.

It is worth mentioning that the method 100 described herein may employrelative signals rather than absolute signals. In some embodiments, theuse of relative signals or ratios can make the particle sizing method100 less sensitive to uniformity fluctuations in the transverseirradiance profile of the light beam illuminating the particles. As aresult, when ratios of scattered signals are used for particle sizing,the beam of light may not need to fulfill specific requirements in termsof uniformity and/or power stability. Also, the location of theilluminated particles may vary within the monitored volume withoutdegrading the validity of the size determination.

Of course, numerous modifications could be made to the embodimentsdescribed above without departing from the scope of the presentinvention.

The invention claimed is:
 1. A system for particle analysis using lightscattering, the system comprising: a plurality of light deflectorsconfigured to deflect light scattered by particles in response toillumination light; an image capture device configured to collectscattered light deflected by the plurality of light deflectors andgenerate from the collected scattered light an image comprising aplurality of sub-images respectively associated with the plurality oflight deflectors, the plurality of sub-images representing the lightscattered by the particles in a respective plurality of scattering angleranges; and a processor configured to receive the image from the imagecapture device, analyze the plurality of sub-images of the image, anddetermine therefrom one or more characteristics of the particles.
 2. Thesystem of claim 1, further comprising an optical source configured togenerate the illumination light to illuminate the particles.
 3. Thesystem of claim 2, wherein the optical source comprises one or aplurality of monochromatic light sources.
 4. The system of claim 1,wherein the processor is configured to identify the particles as spotsin the plurality of sub-images and determine the one or morecharacteristics of the particles from size, shape, location, intensity,polarization, temporal, or spectral information associated with theidentified spots, or a combination thereof.
 5. The system of claim 1,wherein the light deflectors comprise light reflectors.
 6. The system ofclaim 1, wherein the light deflectors comprise optical filters todeflect light scattered by the particles in a spectrally selectivemanner.
 7. The system of claim 1, wherein the image capture device hasspectrally resolved imaging capabilities, temporally resolved imagingcapabilities, or both.
 8. The system of claim 1, wherein the collectedscattered light comprises scattered light that is wavelength-shiftedwith respect to the illumination light.
 9. The system of claim 8,wherein the wavelength-shifted scattered light comprises fluorescentlight.
 10. The system of claim 1, wherein the one or morecharacteristics of the particles comprise composition information. 11.The system of claim 10, wherein the one or more characteristics of theparticles further comprise size, shape, phase, or position information,or a combination thereof.
 12. A method of particle analysis using lightscattering, comprising: illuminating particles with illumination light;deflecting, with a plurality of light deflectors, light scattered by theparticles in response to the illumination light; using an image capturedevice to collect scattered light deflected by the plurality of lightdeflectors, and generating, from the collected scattered light, an imagecomprising a plurality of sub-images respectively associated with theplurality of light deflectors, the plurality of sub-images representingthe light scattered by the particles in a respective plurality ofscattering angle ranges and conveying information indicative of one ormore characteristics of the particles; and analyzing the plurality ofsub-images and determining therefrom the one or more characteristics ofthe particles.
 13. The method of claim 12, wherein analyzing theplurality of sub-images comprises identifying spots associated with theparticles in the plurality of sub-images, and wherein determining theone of more characteristics of the particles comprises assessing size,shape, location, intensity, polarization, temporal, or spectralinformation relating to the spots, or a combination thereof.
 14. Themethod of claim 12, wherein deflecting light scattered by the particlescomprises deflecting light scattered by the particles in a spectrallyselective manner.
 15. The method of claim 12, wherein the collectedscattered light comprises scattered light that is wavelength-shiftedwith respect to the illumination light.
 16. The method of claim 12,wherein the collected scattered light comprises fluorescent light. 17.The method of claim 12, wherein determining the one or morecharacteristics of the particles comprise determining compositioninformation.
 18. The method of claim 17, wherein determining the one ormore characteristics of the particles further comprises determiningsize, shape, phase, or position information about the particles, or acombination thereof.
 19. A system for fluorescence-based particleanalysis, the system comprising: an optical source configured toilluminate particles with illumination light; a plurality of lightdeflectors configured to deflect, in a spectrally sensitive manner,fluorescent scattered light emitted by the particles in response to theillumination light; an image capture device configured to collectfluorescent scattered light deflected by the plurality of lightdeflectors and generate an image from the collected fluorescentscattered light, the image comprising a plurality of sub-imagesrespectively associated with the plurality of light deflectors, theplurality of sub-images representing the fluorescent light scattered bythe particles in a respective plurality of scattering angle ranges andconveying composition information about the particles; and a processorconfigured to receive the image from the image capture device, analyzethe plurality of sub-images of the image, and determine therefrom thecomposition information about the particles.